Purpose: :
Circadian (24-hr) clocks endogenous to the retina regulate most aspects of retinal function. The clock mechanism is cell-based and composed of a highly conserved set of "clock components." In the retina, the clock components have been found in all nuclear layers but the exact phenotype of the cells that express these proteins and whether they do so rhythmically remain largely unknown. Here we examined in detail the types of retinal cells that express the circadian clock core components and determined whether their expression was rhythmic under different lighting conditions.

Results: :
PER1 expression was found in cones, amacrine cells and ganglion cells, but not in horizontal cells or bipolar cells. PER2, CLOCK, and BMAL1 were expressed in cones and all the cells of the inner nuclear and ganglion cell layers. PER1, PER2, CLOCK and CRY2 expression was found rhythmic in cones whereas only PER1 and PER2 showed rhythmic expression in cells of the inner retina. Rhythmic clock protein expression peaked around midday under both light/dark and circadian conditions and was synchronous among retinal cells. Finally, the amplitude of the PER1 and PER2 rhythms were higher under the light/dark cycle than under circadian conditions.

Conclusions: :
Our observations indicate that the composition of the clock mechanism varies among retinal cell types. This variation could reflect different intrinsic rhythmic properties. In particular the coordinated expression of PER1, PER2, CLOCK and CRY2 in cones may reflect more robustness of the clock mechanism in cones than in any other retinal cell type. Our data also show that PER1 and PER2 expression is light sensitive, indicating that these two clock components may be involved in light entrainment mechanisms.